The most common primary malignant brain tumor, glioblastoma (GBM) remains uniformly fatal despite surgical resection, radiation therapy and chemotherapy. Moreover, these conventional therapies are associated with considerable mortality and result in incapacitating damage to surrounding normal brain and system tissues. A promising alternative is the use of immunotherapy, which has ts c in phase III clinical trials, leading to pivotal approvals by the FDA of two prototypic immune-based cancer treatments within the past two years. Substantial evidence suggests that T cells, particularly cytotoxic T lymphocytes (CTLs), have the ability to eradicate large, well-established tumors while leaving healthy cells untouched. Bispecific T cell Engagers (BiTEs) represent an emerging class of bispecific antibody that has been shown to effectively redirect T cells against tumor cells. BiTEs promise to overcome many critical barriers that have traditionally limited translation of immunotherapy to the clinic. Separating this platform from other available immunotherapeutic approaches, our preliminary data support that BiTEs are (1) highly-specific molecules that greatly reduce the risk of toxicity, (2) have the ability to penetrate the BBB and accumulate in intracerebral tumors, and (3) may potentially overcome multiple mechanisms of immunosuppression present in patients with GBM. To date, BiTEs have not previously targeted tumor-specific antigens, and until my studies, have not yet been tested for their ability to mediate activity against tumors of the central nervous system (CNS). Here, I design a BiTE against the EGFRvIII tumor-specific antigen and perform preclinical tests to determine its efficacy against EGFRvIII-expressing glioblastoma (GBM). In this proposal, I will determine critical cellular components of a potent BiTE-mediated antitumor response. Furthermore, I explore the possibility that BiTEs convert suppressive, regulatory T cells into immune cells with potent antitumor effector function. This work has the potential to improve the clinical management of patients with GBM by generating a novel therapeutic.